This invention relates to a multi-point control unit for use in a multi-point videoconference system which has a plurality of videoconference terminals.
As is well known in the art, a multi-point videoconference system is used for holding a videoconference among participants who are in a plurality of places spaced one another. The multi-point videoconference system includes a plurality of videoconference terminals which are individually located in the places and which are connected to one another, for example, through a network. Each of the participants participates in the videoconference through one of the videoconference terminals.
A type of the multi-point videoconference system has a multi-point control unit which is connected to the videoconference terminals through a network. The multi-point control unit receives terminal picture data signals from the videoconference terminals through the network and provides a combined picture data signal that the terminal picture data signals are combined with one another to each of the videoconference terminals through the network. Each of the videoconference terminals displays a combined picture including a plurality of divisional pictures by the use of a split screen technique in response to the combined picture data signal.
A conventional multi-point control unit has a data exchanging unit which is connected to a network to transmit/receive data signals to/from a plurality of videoconference terminals. A plurality of decoders individually corresponding to the videoconference terminals is connected to the data exchanging unit and decodes terminal picture data signals sent from the videoconference terminals to produce decoded terminal picture signals. Furthermore, the decoders thin out parts of the decoded terminal picture signals to produce divisional picture signals. A picture combining unit is connected to the decoders and combines the divisional picture signals to produce a combined picture signal. An encoder is connected between the picture combining unit and the data exchanging unit and encodes the combined picture signal so that each divisional picture signal included in the combined picture signal is encoded by the use of individual quantizing characteristic. The encoder supplies an encoded combined picture data signal to the videoconference terminals through the data exchanging unit and the network.
As mentioned above, the conventional multi-point control unit must carry out both of decoding and encoding to produce the combined picture data signal. This causes large deterioration of quality of the combined picture displayed on each of the videoconference terminals. Particularly, when one of the divisional pictures becomes large about coding data, another divisional picture having small coding data deteriorates in quality.
Such a problem is already indicated in Japanese Unexamined Patent Publications Nos. 2-58486 and 3-6190. Though these Publications teach means of solving the problem, the means require changing the videoconference system in whole.
Besides, similar multi-point control unit is disclosed in U.S. Pat. No. 4,650,929 issued to Boerger et al. However, nothing is mentioned about decoding and encoding in this disclosure.
In addition, various encoders and decoders are disclosed in Japanese Unexamined Patent Publications Nos. 2-105791, 3-10464, 4-176290 and 6-169452. However, though these encoders and decoders change quantizing step for each block of picture elements, they can not handle divisional pictures.